There has so far been known an automatic transmission of a vehicle such as an automotive vehicle and the like each of which is provided with a fluid transmission apparatus represented by a torque convertor capable of transmitting the rotation of an engine to an input shaft of the automatic transmission.
The conventional torque convertor comprises a pump impeller, a turbine runner, and a stator secured to a housing forming part of the automatic transmission through a one-way clutch. The pump impeller, the turbine runner and the stator are each provided with vanes which are soaked in oil, i.e., operating fluid serving as a medium to transmit the torque of an engine to an input shaft of the automatic transmission.
The torque convertor can absorb the torque fluctuation of the engine before transmitting the torque of the engine to the transmission, whereas has a drawback such as transmission loss caused by the operating fluid sealed in the torque convertor, thereby giving rise to the decrease in fuel consumption efficiency.
In view of this drawback, the torque convertor is generally constructed to be assembled with a lock-up clutch capable of integrally connecting the pump impeller and the turbine runner in a high rotation range of the engine smaller in torque fluctuation.
The lock-up clutch of this kind is disposed in face-to-face relationship with a front cover for transmitting the rotation of the engine to the pump impeller. The lock-up clutch is provided with a lock-up piston in a disc shape which is to be pressed against the inner peripheral surface of the front cover by the difference of the pressures of the operating oil in an engagement side oil chamber and a release side oil chamber, so that when the lock-up piston is pressed against and thus brought into engagement with the front cover, the rotation of the engine is directly transmitted to the input shaft of the automatic transmission.
Between the front cover and the lock-up piston is disposed a single plate clutch which has a friction material provided on one side surface of the front cover or the lock-up piston, a sliding surface provided on the other side surface of the front cover or the lock-up piston to face the friction material and slidably contactable with the friction material.
In this type of single clutch having a single sliding surface, it is required to have the area of the sliding surface increased for the purpose of increasing torque capacity of the lock-up clutch mechanism when the lock-up piston is coupled with the front cover. The sliding surface thus increased results in increasing the radial size of the lock-up piston, and thus leading to making the lockup mechanism enlarged in size.
In order to improve the point as previously mentioned, known is another lock-up clutch mechanism which is provided with a multi-plate clutch as shown in FIGS. 7, 8 (for example see Patent Document 1). The known lock-up clutch mechanism 1 is shown in FIGS. 7, 8 to be provided with a lock-up piston 2. Between the lock-up piston 2 and a front cover 3 facing the lock-up piston 2 is provided a multi-plate clutch 4.
The multi-clutch 4 comprises a clutch plate 6 splined to an outer diameter drum portion 5 mounted integrally formed with the front cover 3 and having both axial side surfaces respectively affixed with friction materials 6a, 6b, and a clutch plate 8 splined to a drum portion 7 integrally formed with the lock-up piston 2 and having one axial side surface affixed with a friction material 8a. 
The front cover 3 has an inner peripheral surface having a sliding surface 3a frictionally sliding with the friction material 8a, while the lock-up piston 2 has one side surface having a sliding surface 2a frictionally sliding with the friction material 6b. The clutch plate 8 has one side surface having a sliding surface 8b frictionally sliding with the friction material 6a . The multi-plate clutch 4 is therefore constructed to have three sliding surfaces.
The lock-up clutch mechanism 1 having such a multi-plate clutch 4 can be constructed to have the sliding area enlarged when the lock-up piston 2 is pressed against the front cover 3, so that the torque capacity of the lock-up clutch mechanism 1 can be increased, thereby making it possible to diminish the radial size of the lock-up piston 2, and thereby to downsize the lock-up clutch mechanism 1.
In recent years, it has been paid much consideration that the lock-up clutch mechanism is retained in a connected state as far as possible even in a driving area with a relatively large torque fluctuation of the engine in order to further improve fuel consumption and other efficiencies of the engine.
From this point of view, the lock-up piston and the front cover are not fully integrally connected, but instead are pressed against each other in what is called a half-clutch state to achieve a slip control (see for example Patent Document 2). However, the lock-up clutch mechanism thus constructed is subjected to frictional heat generated by the sliding frictions between the friction materials if there are many opportunities in the slip control that the friction materials are brought into friction engagement with the sliding surfaces.
When the slip control to be performed by the lock-up clutch mechanism 1 as shown in FIGS. 7, 8 is considered, the frictional heat is generated between the friction material 8a and the sliding surface 3a of the front cover 3, between the friction material 6b and the sliding surface 2a of the lock-up piston 2, and between the friction material 6a and the sliding surface 8b of the clutch plate 8.
In general, the friction heat generated between the friction material 8a and the sliding surface 3a of the front cover 3 are easily absorbed by the front cover 3 having large torque capacity, however, the friction heat generated between the friction material 6b and the sliding surface 2a of the lock-up piston 2 is difficult to be absorbed because the heat capacity of the lock-up piston 2 is not so large compared with that of the front cover 3.
Therefore, the multi-clutch 4 encounters such a problem that the heat resistance of the friction material 6b is reduced, and thus is deteriorated earlier. For this reason, it is difficult to expand the slip range and to improve the fuel consumption in a wide driving area.
In order to solve these problems, it may be considered that the friction material is formed with a plurality of cooling grooves extending in the radial direction of the friction material and spaced apart from each other in the circumferential direction of the friction material to ensure that the operating oil introduced into the release side oil chamber from the engagement side oil chamber is allowed to flow in the cooling grooves to cool the friction material when the lock-up piston is pressed against the front cover in the half-clutch state (see for example Patent Document 3).